A conventional bandmill has a pair of spaced, co-planar wheels which carry and drive an endless loop of bandsaw blade. A strain mechanism is provided to force the wheels apart along a longitudinal axis for applying load to, or tensioning the saw, commenly called "straining the saw". Many types of mechanisms for straining the saw have been devised. Some of these mechanisms use mechanical links and a hanging weight system, termed a "dead weight lever mechanism", but this system has inherent limitations mainly due to friction between the various mechanical components and inertia of the weight, both of which contribute to a system having poor sensitivity, a slow response, and poor damping. Sensitivity can be defined as the minimum additional force required to produce a unit increase in actual saw strain in response to a unit increase in strain applied to the saw straining mechanism. With a system having poor sensitivity, it is difficult to know the exact strain applied to the saw, and consequently difficult to take corrective action so as to run the saw at the desired strain. A slow response results in a relatively long response time between a sudden increase in load on the saw blade and the strain system reacting to accommodate the sudden load increase. This slow response tends to result in the saw being over strained, and can occur when the saw is subjected to a lateral force which deflects the saw from its normal cutting plane. Excessive saw strain tends to result in premature failure of the saw, as well as excessive wear of the strain system. Poor damping characteristics result in "overshoot or undershoot" of the strain mechanism, thus running the saw for undesirably long periods away from the desired saw strain.
To prevent a saw from running off the wheels, either one or both of the wheels of a conventional bandmill are "tilted". A wheel is "tilted" by rotating the wheel arbor about an axis disposed normally to the axis of rotation of the wheel, and normally to the longitudinal axis extending between the wheels. A saw is termed as "tracking accurately" if the saw runs on its wheels in a desired location with negligible adjustment during cutting. Accurate tracking is important to avoid excessive saw tooth overhang which reduces accuracy, and in the extreme to avoid loss of the saw from flying off the wheels, causing danger to operators as well as damaging the saw. Insufficient overhang of the saw teeth causes the teeth of the saw to contact the wheels themselves, which damages both the saw and the wheels. In practice, preferably the saw blade passes over the wheels so that the saw tooth gullets between the teeth protrude a small distance, typically 2 mm to 5 mm beyond the edges of the wheels. Faces of the wheels carrying the saw are commonly "crowned", which provides a raised center portion of the face of the wheel which assists in tracking of the saw. Also, most saws are "roll tensioned" which is a pre-stressing treatment of the saw prior to installation on the bandmill. Roll tensioning is designed to ensure that the cutting or leading edge portion of the saw is always maintained at a higher tension than the center and/or trailing edge portion of the saw. By maintaining the cutting edge of the saw at a higher tension than remaining portions of the saw, heat produced during cutting does not cause the leading edge of the saw to "slacken", that is to lose tension so that the leading edge operates at a lower tension than remaining portions of the saw. If the leading edge of the saw slackens during use, sawing accuracy decreases, and the saw has a greater tendency to lose its track on the wheels, increasing danger to operators and risk of damage to the equipment.
In order to maintain accurate tracking of the saw, at least one wheel is tilted so that the axis of rotation of that said one wheel is shifted relative to the axis of rotation of remaining wheel. Normally, with an ideal saw, axes of arbors of the wheels mounting the saw will be parallel to each other, and within a common longitudinal plane, which is parallel to a cutting portion of the saw blade. When one wheel is tilted, its axis is shifted so as to be non-parallel to the remaining axis, and yet remains within the common longitudinal plane containing the remaining axis. Wheel tilting is usually effected by moving one end portion of the movable wheel arbor away from, or towards, the remaining wheel arbor, thus shifting the axis of the wheel.
Both setting the strain of the saw, and adjusting tilt of the wheel so that the saw maintains accurate tracking requires manual adjustment by a bandmill operator. With some strain mechanisms, actual saw strain cannot be accurately or easily determined, due to the inherent limitations of the strain mechanism. With other strain mechanisms, the applied strain can be varied considerably by sawing conditions, for example, fluctuations in saw load due to presence of knotholes, frozen lumber, etc. which tend to deflect the saw from its normal cutting plane. These irregularities increase strain on the saw, and it is virtually impossible for the operator to accommodate sudden strain changes by adjusting the applied strain. Likewise, if the saw becomes excessively heated, and tracking of the saw deteriorates such that corrective action is required, it can be difficult for an operator to respond sufficiently quickly to manually adjust the wheel tilt to reduce loss of tracking accuracy.
From the above discussion, it can be seen that the bandmill operator is required to adjust the strain mechanism to maintain saw strain within a desired strain range, together with adjusting wheel tilt to maintain accuracy of tracking of the saw on the wheels. These adjustments are performed manually and require care and skill, and sometimes require additional adjustment.
All bandmills known to the inventors are relatively wide and can only be used in a specific disposition, i.e. as a vertical, horizontal or slanting bandmill, and this disposition cannot be changed. When conventional bandmills are placed side by side, they require a considerable space and length of feed conveyor which increases cost of multiple bandmill installations. Also, if a sawmill operator wishes to change a vertical bandmill to a horizontal bandmill, it would be essentially impossible, and an alternative bandmill would be required. This reduces flexibility of sawmill plant layout.